Massive black hole might have spontaneously ‘flipped’ its magnetic field

In March 2018, the All-Sky Automated Survey for Supernovae alerted astronomers that a galaxy called 1ES 1927+654 had brightened by nearly 100 times in visible light. A further search showed that the eruption had begun months earlier, towards the end of 2017.

When NASA’s Swift space observatory, which studies gamma-ray bursts, X-ray, UV and visible light, first examined the galaxy in May 2018, its UV emissions were elevated 12 times but kept steadily declining, indicating that there was an earlier unobserved peak. In June, the galaxy’s higher-energy X-ray emission disappeared.

Big galaxies often host supermassive black holes at their centre. When matter falls towards these black holes, it collects into a vast flattened structure called the accretion disk. The material slowly spirals inwards, heats up and emits visible, UV and lower-energy X-ray light. A cloud of extremely hot particles near the black hole, called the corona, produces higher-energy X-rays. The brightness of these emissions from the black hole depends on how much material streams towards it.

“An earlier interpretation of the eruption suggested that it was triggered by a star that passed so close to the black hole it was torn apart, disrupting the flow of gas. We show that such an event would fade out more rapidly than this outburst,” said co-author Josefa Becerra González, of the Canary Islands Institute of Astrophysics, in a NASA press statement.

The disappearance of the higher-energy X-ray emissions in this case gave astronomers an important clue: they suspect that the black hole’s magnetic field creates and sustains the corona–a cloud of extremely hot particles– so any magnetic change could impact its X-ray properties.

“A magnetic reversal, where the north pole becomes south and vice versa, seems to best fit the observations. The field initially weakens at the outskirts of the accretion disk, leading to greater heating and brightening in visible and UV light,” said co-author Mitchell Begelman, a professor in the department of astrophysical and planetary sciences at the University of Colorado Boulder in a press statement. Begelman and his colleagues at the University of Colorado developed the magnetic model.

As the flip happens, the magnetic field becomes so weak that the black hole can no longer support the corona, leading to the X-ray emissions vanishing. Slowly, the magnetic field begins strengthening in its new orientation. In October 2018, about four months after they disappeared, the X-rays came back. This indicated that the corona has been fully restored. By the summer of 2021, the galaxy had completely returned to its pre-eruption state.

According to NASA, magnetic reversals are likely to be common events in the universe as geological records show that the Earth’s field flips unpredictable, reversing a few times every million years in the recent past. The Sun undergoes a magnetic reversal much more often as part of its normal cycle of activity. It switches its north and south poles roughly every 11 years.